In a solar collector of the evacuated tubular type, of which there are many examples, the goal of increasing the efficiency of the collector, even by as a small a degree as a fraction of a percent, is now recognized as important in the state of the art.
The evacuated tubular solar collector poses one of the best possibilities for very high solar efficiency, mainly because the absorber is isolated from ambience by an evacuated jacket or envelope which utilizes a relatively high quality vacuum as an insulating space. Some models of collectors provide for a double wall, concentric tube, vacuum bottle arrangement with the outer tube being a solar window and the inner tube functioning as an absorber, whereas other arrangements utilize a single wall tubular member for a window or envelope with a flat plate or heat pipe absorber surrounded thereby in an evacuated space. Still others diverge from the tubular concept, and provide a flat window, evacuated on one side, with a honeycomb support structure for supporting the window against the atmospheric pressure exerted against the vacuum side. Even still another type uses evacuated tubes for a window in a flat plate arrangement. This latter type may be fabricated with an insulating structure of evacuated tubes or a foam insulating support structure for an absorber.
The aforementioned arrangements are but some of many possible examples available in the art. However, it should be realized that, as previously mentioned, the suppression of thermal losses like convection, conduction, and radiation significantly increases the efficiency of the evacuated collector structures. For example it is known that selective absorbers increase adsorption and reduce emission of energy by reradiation from the absorber surface. Absorptivity .alpha. characteristics of at least as high as 0.9 have been produced, and emissivity characteristics .epsilon. of as low as 0.1 have been achieved. At various stages of development, production cost versus effectiveness of the selective absorber surface so produced is an important factor which most usually determines its ultimate application. Thus a low cost highly effective selective coating for an absorber is envisioned by the present invention.
Another method of reducing loss from the collector, having a selective absorber surface is to coat the collector window to further suppress the emissivity losses from the absorber.
It should be noted that absorbtivity .alpha. and emmissivity .epsilon. are measured in different wavelength regions. Visible and near visible solar radiation from about 0.2 to about 2.0 micrometers is the proper domain for evaluating absorber performance in terms of adsorptivity .alpha., whereas the infrared and near infrared domain of about 2.0 to about 20 micrometers is the characteristic region for measuring the effectiveness of the absorber emissivity .epsilon.. It is known that certain coatings or films are transparent to solar radiation and opaque to infrared energy and vice versa. Thus it is possible to produce an energy trap. As low as losses are, however, there is room for improvement in terms of cost versus efficiency, since small improvements in the reduction of emissivity or increases in absorptivity usually are expensive and may not be cost effective.
It is to this end of providing a more efficient solar collector which is cost effective that the present disclosure is directed.
It has been found also that the possibility of solar system shutdown can occur even though there is available a high solar flux, if the collector is covered with snow. The present invention uses thermal and electrical properties of certain coatings to further increase the effectiveness of such collectors by reducing the possibility of such shutdowns by an effective snow removal element which thereby increases the energy collection time of the absorber.
Preferred coatings, hereinafter described, may be formulated and applied to a substrate in accordance with the principles set forth and described in U.S. Pat. No. 2,564,706 to Mochel, and in a patent application of E. M. Griest filed the same date of the present invention, both references being assigned to Corning Glass Works the assignee herein.